The goal of this proposal is to characterize the mechanism of telomerase-independent telomere maintenance. Although telomerase activation is the most common mechanism for maintenance of telomeric DNA, a subset of tumors and cell lines use a telomerase-independent mechanism, termed Alternative Lengthening of Telomeres (ALT), to sustain telomeric DNA. In telomerase-deficient yeast strains, telomere stabilization is achieved through recombination-based mechanisms. Similarly, although the molecular mechanism has yet to be established, the evidence from mammalian systems is consistent with ALT involving recombination. Based on data from yeast studies, it has been suggested that telomere recombination requires loss of the capping activity by which telomeres were originally defined. To test this hypothesis in mammalian cells, the structure of ALT telomeres will be determined using molecular biological methods and electron microscopy. Clarifying the structure of ALT telomeres will provide insight into the molecular mechanism of ALT. Secondly, factors involved in DNA recombination that are essential for telomere maintenance by ALT will be identified using dominant negative and knockdown approaches. These data will characterize the molecular requirements for ALT and will differentiate between possible recombination-based mechanisms. The only genetic alteration in mammalian cells linked to a predisposition to activate ALT is p53 status. Among its other functions, p53 has been shown to be involved in suppression of recombination. Thus, the role of p53 in regulating ALT will be characterized using temperature sensitive alleles of p53. These data will be the first that illustrate how alterations in a factor might predispose cells to ALT. Inhibition of telomerase has become an attractive target for future chemotherapeutic agents. However, ALT tumors would be refractory to these treatments. This proposal will identify structural changes and factors essential for activation of ALT and thus identify new potential targets for chemotherapeutic agents.